But Glen Johnson, a political editor with Boston.com, says that because federal election law bars candidates from coordinating with political action committees and other outside groups, it's not clear how much power the two have to do so.

"This is completely separate money being spent by third party group that in a way have a fiduciary responsibility to their donors to spend money on these ads in the way they see fit," Johnson said. "So Elizabeth Warren and Scott Brown can’t practically call on anybody to stop this."

Republican Charles Baker, who ran for governor against incumbent democrat Deval Patrick in 2010, says it’s a question of enforceability.

"Let’s suppose it’s October 1st, right? And it’s 47-47 with 5 percent undecided or something. Are people going to be able to resist? Are the outside forces going to be able to resist the urge to try to lay it out on the other guy? I don’t know. I mean the question becomes whether the candidates have the ability to kinda shout ‘em down," Baker said.